The present application claims the Convention priority from Japanese Patent Application No. 2001-304033, the complete disclosures of which are hereby incorporated herein by reference.
1. Field of the Invention
The present invention relates to a light emitting element. More particularly, it relates to a light emitting element in which surface irregularities are formed on at least a side surface of a light emitting region formed on a substrate, and light is radiated from the side surface so as to increase the external quantum efficiency of the light emitting element.
The present invention can be applied to a light emitting element such as an LED which requires an increased external quantum efficiency.
2. Related Art
From in the past, there have existed semiconductor light emitting elements in which irregularities are formed on the top surface of a light emitting element to increase the light emitting efficiency. Examples of such light emitting elements are the semiconductor light emitting element disclosed in Japanese Patent Application Laid-Open (kokai) No. 5-167101 and the semiconductor light emitting element and manufacturing method therefor disclosed in Japanese Patent Application Laid-Open (kokai) No. 2000-196152. The semiconductor light emitting element disclosed in Japanese Patent Application Laid-Open (kokai) No. 5-167101 is shown in FIG. 11. The semiconductor light emitting element shown in FIG. 11 comprises a compound semiconductor substrate 1 made of n-GaAs or the like, a light reflecting layer 6, light emitting layers 20, a current dispersing layer 3, a light scattering layer 10, and electrodes 4 and 5. The light emitting layers 20 comprise an n-InAlP layer 21, an InGaAlP layer 23, and a p-InAlP layer 22.
The invention disclosed in that publication is characterized in that the light scattering layer 10, which is made of GaP or the like, is formed on the current dispersing layer 3. The light scattering layer 10, which is formed by incomplete growth, has a different lattice constant from the current dispersing layer 3. As a result, total reflection by the surface (the interface with the air) does not occur, and thus light is output with approximately two times the efficiency compared to when light is output from the glossy surface of the current dispersing layer 3.
The semiconductor light emitting element disclosed in Japanese Patent Application Laid-Open (kokai) No. 2000-196152 is shown in FIG. 12. It has a structure in which an n-GaN layer 52, an InGaN light emitting layer 53, and a p-GaN layer 54 are stacked atop a sapphire substrate 51, an electrode 55 is formed atop a portion of the p-GaN layer 54, and an electrode 56 is formed atop a portion of the n-GaN layer 52. The light emitting element is characterized in that the surface 54a of the p-GaN layer 54 is formed to have a plurality of cylindrical lenses. If the surface of the p-GaN layer 54 were made flat in the conventional manner, of the light emitted by the InGaN light emitting layer 53, only the light which fulfilled certain conditions (i.e., light incident within a critical angle of about 21.9xc2x0 centered on a line normal to the surface) would be radiated to the exterior, and the other incident light would be confined by total reflection and attenuated.
However, in that invention, a plurality of cylindrical lenses are formed on the surface of the p-GaN layer 54, so the conditions determining which light can be radiated are relaxed. Thus, the radiation efficiency is increased by making the surface cylindrical.
However, neither of the above prior art examples takes any step with respect to light which is propagated lateral within the light emitting layers. As shown in FIG. 13A and FIG. 13B which are respectively a schematic vertical cross-sectional view and a plan view of a typical semiconductor light emitting element having a rectangular shape, no matter how many times light is reflected within the layer, there is a large amount of light which has an angle of incidence larger than the critical angle, and this light continues to be internally reflected, causing a decrease in the external quantum efficiency of the light emitting element.
In the semiconductor light emitting element disclosed in Japanese Patent Application Laid-Open (kokai) No. 2000-196152, forming cylindrical irregularities on the surface of a light emitting layer does in fact increase the external quantum efficiency, but the layer having the cylindrical irregularities is thin, and therefore, it is difficult to accurately form the irregularities thereon. In addition, with a stable material such as GaN, it is not possible to form random irregularities on the top surface of the material by surface treatment such as chemical etching, which was conventionally carried out for GaP and similar materials. On the other hand, physical methods of forming the cylindrical irregularities are difficult and suffer from poor productivity.
The present invention was made in order to solve the above-described problems. One object of the present invention is to increase the area of a crystal interface of a light emitting element without changing the light density within the crystal; i.e., without changing the size of the crystal, and to thereby increase the external radiation area with respect to emitted light present at random locations and having random orientations within the crystal, and to increase the efficiency of light radiation; i.e., to increase the external quantum efficiency. Another object of the present invention is to achieve such an increase in external quantum efficiency by a simple method so as to permit mass production of an improved light emitting element.
Yet another object of the present invention is to provide a light emitting element having surface irregularities on a side surface and on a top surface of a light emitting element so as to further increase the external quantum efficiency of the light emitting element.
A still further object of the present invention is to provide a light emitting element having a tapered side formed with surface irregularities so as to increase the light emitting efficiency of the light emitting element in the direction perpendicular to a substrate of the light emitting element.
While the various aspects of the present invention can collectively achieve all of the above objects, it should be understood that a single aspect does not necessarily achieve all of the above objects.
According to one form of the present invention, a light emitting element comprises a solid light emitting element having a light emitting region comprising at least one layer, with at least a portion of a side surface of the light emitting region having surface irregularities thereon.
The light emitting region may comprise a single layer or a plurality of layers. When the light emitting region comprises a semiconductor, the semiconductor may be either a p-type or an n-type. When the light emitting region comprises a plurality of layers, the different layers may contain the same compositional proportions having different impurity concentrations in each other, or the layers may differ in compositional proportion or in constituent element. In the latter case, the concentration of added impurities may differ among the plurality of layers. The light emitting region may be non-doped, or it may be an n-type or p-type semiconductor. Layers having various functions may be provided above or below the light emitting region. Such layers can be n-layers, p-layers, or non-doped layers. When the light emitting element is a semiconductor light emitting element, it may employ various structures, such as a homo pn structure, a single hetero structure, or a double hetero structure. The light emitting region can employ a single quantum well structure, a multiple quantum well structure, or the like.
The present invention can be applied not only to an injection type LED but to an intrinsic EL.
In this form of the invention, surface irregularities are on at least a side surface of the light emitting region, but surface irregularities can also be formed on a side surface of regions other than the light emitting region. The surface irregularities can be formed around all or a portion of the periphery of the light emitting region or other region. The larger the portion of the periphery on which surface irregularities are formed, the greater is the effect of the surface irregularities on discharging light. Some examples of situations in which surface irregularities are formed on only a portion of the periphery of a light emitting element are when they are formed on a single side or on opposite sides of a rectangular light emitting element.
The surface irregularities can have a variety of shapes. In one form of the present invention, the surface irregularities are curves having a varying curvature. In this case, in a horizontal cross section the irregularities is a curved line. One example of a curved shape is a curved pillar. The curved pillar may be sloping to the perpendicular direction to the substrate.
In one form of the present invention, the light emitting element is tapered in the light emitting region so that the side surface having surface irregularities is non-perpendicular with respect to a substrate.
The surface irregularities can be formed by a variety of methods. In one form of the present invention, the surface irregularities are formed by etching.
According to another form of the present invention, the surface irregularities are formed by patterning of the light emitting region.
According to yet another form of the present invention, the surface irregularities are formed when a plurality of light emitting elements are separated from each other.
In a preferred embodiment, the light emitting element is formed on a substrate, and the refractive index of the substrate is smaller than the refractive index of the light emitting region having one or more layers.
A low temperature growth buffer layer or a high temperature growth buffer layer may be formed atop the substrate. These buffer layers may comprise a plurality of layers. A buffer layer and a monocrystal layer may alternate with each other.
The substrate may comprise electrically conductive materials or electrically insulating materials. When an electrically conductive material is used to form the substrate, two electrodes can be formed on opposite sides of the substrate. When an electrically insulating material is used to form the substrate, both electrodes are formed on the top surface of the substrate.
According to one form of the present invention, each layer formed on the substrate comprises a Group-III nitride compound semiconductor.
Surface irregularities may be formed not only on a side surface of the light emitting region but may also be formed on the top surface of the light emitting element.
The benefits of the above-described various forms of the present invention will next be described.
As stated above, according to one form of the present invention, a light emitting element comprises a solid light emitting element having a light emitting region comprising at least one layer, with at least a portion of a side surface of the light emitting region having surface irregularities thereon.
Normally, in a light emitting element, light which is generated within a light emitting region is radiated in all directions, but only light in the direction approximately perpendicular to the substrate is radiated in the perpendicular direction to the substrate. Of that light from the light emitting region which is incident on layers above it, such as a p-layer when the light emitting element is a semiconductor light emitting diode (a p-layer when a p-layer is formed above the light emitting region and an n-layer when an n-layer is formed above the light emitting region), that light which is incident within a critical angle is radiated in the direction perpendicular to the substrate. However, since the p-layer (such as a p-GaN layer) and the light emitting region have a refractive index which is higher than the exterior region, light having an angle of incidence which is larger than the critical angle is totally reflected by the p-layer and returns to the light emitting region. Thus, the greater portion of the light is confined within the light emitting region and is attenuated. Therefore, the light which was generated within the light emitting region of a conventional light emitting element was not all efficiently radiated.
However, according to one form of the present invention, surface irregularities are formed on at least a portion of a side surface of the light emitting region. If a side surface of the light emitting region has an irregular shape, normal lines to the side surface extend in various directions. Varying the direction of normal lines to the side surface causes the critical angle to also vary. As a result, light which would have been confined within the light emitting region and particularly light which would have been confined in the lateral direction can be efficiently radiated, and a large portion of light which conventionally could not be radiated can be radiated to the exterior of the light emitting element, so the external quantum efficiency of the light emitting element can be greatly increased. The surface irregularities can have any desired shape. For example, they can have the shape of triangular waves or sinusoidal waves. The shape and the size may vary randomly, or they may be periodic.
As stated above, the surface irregularities may be curves having a varying curvature; i.e., the radius of curvature of the surface irregularities may vary. By having a varying curvature, the critical angle is not fixed as in the case of a flat surface, and thus the critical angle varies, and the overall amount of light which can be radiated from the light emitting element is increased. This is equivalent to increasing the effective radiation area of light. In this manner, light which is generated in the light emitting region can be efficiently radiated to the exterior of the light emitting region, whereby a light emitting element having a higher external quantum efficiency is obtained.
As stated above, the light emitting element may be tapered in the light emitting region so that the side surface having surface irregularities is non-perpendicular with respect to a substrate. If normal lines to the side surface and normal lines to the top surface of the light emitting region are not perpendicular to each other, the area of the side surface can be increased. Accordingly, light which was internally reflected in the lateral direction can be more efficiently radiated to the exterior of the light emitting element. If normal lines to the side surface have a positive angle of slope, light which is radiated from the side surface has a positive angle of slope. Due to the positive angle of slope, the light which is radiated from the side surface is effectively radiated in the upwards direction. Alternatively, the radiated light may have a negative angle of slope. In this case, light which is radiated from the light emitting region reflects off the surface of a substrate on which the light emitting region is formed and then is again radiated upwards.
As stated above, one possible method of forming the surface irregularities is by etching.
When a light emitting element employs an electrically insulating substrate, an n-type layer is formed on the bottom surface. In the case of an element structure in which a positive and negative electrode are formed on the upper side of the substrate, it is necessary to dig down by etching in each layer in order to form an electrode in the n-type layer. At this time, an irregular pattern can be formed on the top surface by a mask, a resist or the like. If digging is performed through each layer by etching with the pattern formed in this manner, irregularities in the shape of an etching pattern are formed on the side surface of the light emitting region to form an irregular side surface. The irregular side surface is formed by etching, so it can be easily formed.
As stated above, another method of forming the surface irregularities in a side surface of the light emitting region is by patterning of the light emitting region.
For example, if the surface irregularities are formed by patterning on a light emitting region formed by organic metal compound vapor phase growth, the shape of the surface irregularities can be chosen at will, and a wide variety of shapes are possible, such as triangular wave shapes, sine wave shapes, and random shapes. In addition, patterning makes it possible to strictly control the shape of the surface irregularities such that light which is radiated from any point on the side surface is not blocked by another of the surface irregularities. Accordingly, the shape of the side surface can be optimized, and light can be more efficiently radiated.
As stated above, the surface irregularities can also be formed as part of the process of separating a plurality of light emitting elements from each other. In this case, surface irregularities can be easily formed on the side surface without the need for a separate and independent step of forming irregularities, and thus surface irregularities can be formed with no increase in manufacturing costs.
As stated above, a light emitting element according to the present invention can be formed atop a substrate, with the refractive index of the substrate being smaller than the refractive index of the light emitting region.
If the refractive index of the substrate is smaller than the refractive index of the light emitting region, it becomes easy to obtain total reflection from the substrate, and a portion of the light which is generated in the light emitting region is propagated in the lateral direction within the light emitting region. For example, in view that a GaN-type semiconductor layer has a refractive index of approximately 2.4, an alumina-group substrate having a refractive index of 1.7 can be used. As a result, light emitted by the light emitting region is totally reflected by the substrate, and it reaches the side surface of the light emitting region, from where it is radiated. Therefore, a light emitting element having an even higher external quantum efficiency can be obtained.
As stated above, each layer formed on the substrate may comprise a Group-III nitride compound semiconductor.
When a Group-III nitride compound semiconductor is used to form a light emitting element, it is a direct transition type semiconductor having a wide emission spectrum ranging from ultraviolet to red. Accordingly, it can be used to manufacture light emitting diodes (LED""s) having various emission spectra.
The band gap of a Group-III nitride compound semiconductor is broad, so a light emitting element can be achieved which can stably operate at higher temperatures than elements using other types of semiconductors. In addition, a Group-III nitride compound semiconductor does not use arsenic (As) as a main component, and therefore, a light emitting element can be achieved which is safe from an environmental standpoint.
As stated above, a light emitting element according to the present invention may have surface irregularities formed not only on a side surface of its light emitting region but also on its top surface.
Irregularities are effective not only on the side surface of the light emitting region but also on the top surface of the light discharging surface. Surface irregularities on the top surface can increase the effectiveness of light radiation from the top surface for the same reasons that surface irregularities on a side surface can increase the effectiveness of light radiation from the side surface. Light can be more effectively radiated from the side surface and the top surface; therefore, by forming surface irregularities on the top surface, a light emitting element can be obtained with further increased quantum efficiency.